4.0 Geometric and Safety Design

4.7.2 Recommended Guidelines

Vertical Profile Grade Design Issue and Guideline Development

Construction of a toll plaza at the crest of a profile grade results in sight distance advantages and plaza operations benefit from gravitational forces in slowing vehicles approaching the toll lanes and accelerating vehicles departing the plaza. Consequently, some studies have recommended the use of a + 3% grade for the plaza approach and departure area. Unfortunately, when the plaza’s mixed flow traffic includes commercial vehicles, a 3% grade will adversely affect the performance of these vehicles, resulting in additional delays through the plaza. A vertical profile grade greater than or equal to + 1% and less than or equal to + 2% better accommodates the performance of commercial vehicles under the stop and go conditions normally encountered in plaza queue zones. For the toll lanes, the cross slope and the vertical profile grade should be designed concurrently to assure proper drainage. Under no circumstances should the vertical profile grade be less than + 0.5% or exceed + 2% in a toll lane. This avoids the undesirable need to install trench or slot drains across the toll lane entrance that may clog, causing the possible unsafe condition (to both attendant and user) of ice formation within the lane. The canopy and storm drainage system design must direct collected water away from the toll lanes.

Recommended Guidelines

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